The present invention relates to a flat cell.
The prior art related to the present invention includes a technique of a flat cell in which laminated electric-power-generating elements are held between a pair of sheet-like metal terminal plates having a predetermined shape, and an annular insulating member is interposed between the respective peripheral edges of these terminal plates so that the electric-power-generating element is hermetically sealed. In a cell which employs an active light metal such as lithium as a negative active material, lithium and water vapor react with each other to generate hydrogen gas, causing the internal pressure of the cell to rise. Therefore, the above-described annular insulating member must be designed so as not to become defective as a result of such rise in the cell internal pressure. In Japanese Unexamined Patent Publication No. 83340/1984, maleic anhydride modified polyethylene resin is employed as the annular insulating member and is bonded to the terminal plates by hot-plate pressure bonding, inpulse bonding, ultrasonic bonding, etc. In this prior art, a separator which is made from polypropylene nonwoven fabric is interposed between the positive active material and the negative active material, i.e., Li.
The use of only maleic anhydride modified polyethylene resin as a sealing member, however, has led to short-circuiting between a positive terminal plate and a negative terminal plate when the peripheral portion of the cell is heat-sealed under pressure.
Further, since it is difficult to make uniform the thickness of the heat-sealed peripheral portion of the cell, the prior art has the disadvantage of poor sealing properties of the cell. Because the sealing member is heat-sealed under pressure at a melting point or more, there may be variations in the thickness of the sealing member along the periphery of the cell.